Default Linux ..... not with the number of distributions all with their own theme, default for a particular distro perhaps
Apart from Firefox you can add a number of browsers to Mint if you like and decide on your default

As Husse said, there aren't necessarily defaults, but there are browsers developed with Gnome and KDE that fit in better than say Firefox would. For Gnome, Epiphany is the browser and for KDE it's Konqueror.